Gene expression of the human plasma membrane-associated sialidase (NEU3), a key enzyme for ganglioside degradation, is relatively high in brain and is modulated in response to many cellular processes, including neuronal cell differentiation and tumorigenesis. We demonstrated previously that NEU3 is markedly up-regulated in various human cancers and showed that NEU3 transgenic mice developed a diabetic phenotype and were susceptible to azoxymethane-induced aberrant crypt foci in their colon tissues. These results suggest that appropriate control of NEU3 gene expression is required for homoeostasis of cellular functions. To gain insights into regulation mechanisms, we determined the gene structure and assessed transcription factor involvement. Oligo-capping analysis indicated the existence of alternative promoters for the NEU3 gene. Transcription started from two clusters of multiple TSSs (transcription start sites); one cluster is preferentially utilized in brain and another in other tissues and cells. Luciferase reporter assays showed further that the region neighbouring the two clusters has promoter activity in the human cell lines analysed. The promoter lacks TATA, but contains CCAAT and CAAC, elements, whose deletions led to a decrease in promoter activity. Electrophoretic mobility-shift assays and chromatin immunoprecipitation demonstrated binding of transcription factors Sp (specificity protein) 1 and Sp3 to the promoter region. Down-regulation of the factors by siRNAs (short interfering RNAs) increased transcription from brain-type TSSs and decreased transcription from other TSSs, suggesting a role for Sp1 and Sp3 in selection of the TSSs. These results indicate that NEU3 expression is diversely regulated by Sp1/Sp3 transcription factors binding to alternative promoters, which might account for multiple modulation of gene expression.

We have found previously that human plasma-membrane-associated sialidase (NEU3), a key glycosidase for ganglioside degradation, was markedly up-regulated in human colon cancers, with an involvement in suppression of apoptosis. To elucidate the molecular mechanisms underlying increased NEU3 expression, in the present study we investigated its role in cell adhesion of human colon cancer cells. DLD-1 cells transfected with NEU3 exhibited increased adhesion to laminins and consequent cell proliferation, but decreased cell adhesion to fibronectin and collagens I and IV, compared with control cells. When triggered by laminins, NEU3 clearly stimulated phosphorylation of FAK (focal adhesion kinase) and ERK (extracellular-signal-regulated kinase), whereas there was no activation on fibronectin. NEU3 markedly enhanced tyrosine phosphorylation of integrin β4 with recruitment of Shc and Grb-2 only on laminin-5, and NEU3 was co-immunoprecipitated by an anti-(integrin β4) antibody, suggesting that association of NEU3 with integrin β4 might facilitate promotion of the integrin-derived signalling on laminin-5. In addition, the promotion of phosphorylation of integrin β1 and ILK (integrin-linked kinase) was also observed on laminins. G M3 depletion as the result of NEU3 overexpression, assessed by TLC, appeared to be one of the causes of the increased adhesion on laminins and, in contrast, of the decreased adhesion on fibronectin – NEU3 probably having bimodal effects. These results indicate that NEU3 differentially regulates cell proliferation through integrin-mediated signalling depending on the extracellular matrix and, on laminins, NEU3 did indeed activate molecules often up-regulated in carcinogenesis, which may cause an acceleration of the malignant phenotype in cancer cells.

Based on the human cDNA sequence predicted to represent the NEU4 sialidase gene in public databases, a cDNA covering the entire coding sequence was isolated from human brain and expressed in mammalian cells. The cDNA encodes two isoforms: one possessing an N-terminal 12-amino-acid sequence that is predicted to be a mitochondrial targeting sequence, and the other lacking these amino acids. Expression of the isoforms is tissuespecific, as assessed by reverse transcription–PCR. Brain, muscle and kidney contained both isoforms; liver showed the highest expression, and the short form was predominant in this organ. In transiently transfected COS-1 cells, enzyme activity was markedly increased with gangliosides as well as with glycoproteins and oligosaccharides as substrates compared with the control levels. This differs from findings with other human sialidases. Although the isoforms were not distinguishable with regard to substrate specificity, they exhibited differential subcellular localizations. Immunofluorescence microscopy and biochemical fractionation demonstrated that an exogenously expressed haemagglutinin-tagged long form of NEU4 was concentrated in mitochondria in several human culture cell types, whereas the short form was present in intracellular membranes, indicating that the sequence comprising the N-terminal 12 amino acid residues acts as a targeting signal for mitochondria. Co-localization of the long form to mitochondria was further supported by efficient targeting of the N-terminal region fused to enhanced green fluorescent protein, and by the targeting failure of a mutant with an amino acid substitution in this region. NEU4 is possibly involved in regulation of apoptosis by modulation of ganglioside G D3 , which accumulates in mitochondria during apoptosis and is the best substrate for the sialidase.

In mammalian tissues, the pathway known for the catabolism ofG M1 [Galβ3GalNAcβ4(Neu5Acα3)Galβ4GlcCer;where Cer is ceramide] is the conversion of this ganglioside into G M2 [GalNAcβ4(Neu5Acα3)Galβ4GlcβCer] by β-galactosidase followed by the conversion of G M2 into G M3 (Neu5Acα3Galβ4GlcβCer) by β-N-acetylhexosaminidase A (Hex A). However, the question of whether or not G M1 and G M2 can also be respectively converted into asialo-G M1 (Galβ3GalNAcβ4Galβ4GlcCer; G A1 ) and asialo-G M2 (GalNAcβ4Galβ4GlcβCer, G A2 ) by mammalian sialidases has not been resolved. This is due to the fact that sialidases purified from mammalian tissues always contained detergents that interfered with the in vitro hydrolysis of G M1 and G M2 in the presence of an activator protein. The mouse model of human type B Tay–Sachs disease created by the disruption of the Hexa gene showed no neurological abnormalities, with milder clinical symptoms than the human counterpart, and the accumulation of G M2 in the brains of affected mice was only limited to certain regions [Sango, Yamanaka, Hoffmann, Okuda, Grinberg, Westphal, McDonald, Crawley, Sandhoff, Suzuki and Proia (1995) Nat. Genet. 11 , 170–176]. These results suggest the possible presence of an alternative catabolic pathway (the G A2 pathway) in mouse to convert G M2 into G A2 by sialidase. To show the existence of this pathway, we have used recombinant mammalian cytosolic sialidase and membrane-associated sialidase to study the desialylation of G M1 and G M2 . We found that the mouse membrane-bound sialidase was able to convert G M1 and G M2 into their respective asialo-derivatives in the presence of human or mouse G M2 activator protein. The cytosolic sialidase did not exhibit this activity. Our results suggest that, in vivo , the stable NeuAc of G M1 and G M2 may be removed by the mammalian membrane-associated sialidase in the presence of G M2 activator protein. They also support the presence of the G A2 pathway for the catabolism of G M2 in mouse.